In alt.engineering.electrical wrote:
+----+ +---|AC +DC|--+--7809---+---+-------+
| | | | | | +|C2 | __|__
)||( | | +|C1 | === R1 | + |--+Sense
120 )||(12 |Rect. | === | | | |Opto |
Vac )||(Vac| | | | +|C3 P-+ |__-__|--+Loop
)||( | | | | === | | |
| | | | | | | | | |
+----+ +---|AC_-DC|--+---+-----+---+--+----+

C1 is a 1000 uf 25 volt cap. C2 and C3 are .1 farad 5 volt super caps.
(Allelectronics sells these for 85 cents each)

The 85 cent ones (CBC-125) are 1 F, 2.5 V. You'd need four in series to
handle 9 V. The manufacturer may recommend a resistor in parallel with
each cap, which I think makes a voltage divider to keep the voltage
about the same on each cap. Digi-Key has Cooper PowerStor capacitors
that come already packaged as two 2.5 V caps in series to get a 5 V
rating for $4.20 to $10 quantity 1.

Your second idea, using the CBC-11/CBC-12 1 F 5.5 V memory back-up
capacitors, may or may not work. Some of these capacitors are made for
very low current (microamps, CMOS memory) and not the tens to hundreds
of milliamps you'd need for a relay or the 10-20 milliamps you'd need
for an optoisolator or solid state relay.

Matt Roberds